1. Field of the Invention
The present invention relates to an orthogonal spread code of a CDMA mobile communication system, and more particularly, to a method for generating an LS code via extension of the initial matrix based upon certain rules.
2. Description of the Related Art
In general, a Code Division Multiple Access (CDMA) mobile communication system adopts a spread spectrum communication method which uses a spread code with a transmission bandwidth which is much wider than that of an information signal to be transmitted. The spread spectrum communication method uses a wide frequency bandwidth, and thus can regenerate an original signal via after despreading which increases the signal power and keeps the noise power low. According to a basic principle of the spread spectrum communication method, when a transmitting block modulates a data multiplied by a spread code to widen the bandwidth of a frequency and then transmits a signal, a receiving block multiplies the signal by the same spread code used in the transmitting block to narrow the bandwidth of the frequency and then demodulates the signal to detect the original signal. In general, the signal received through an antenna of the receiving block includes several kinds of noises mixed thereto in addition to the original signal. However, using the spread spectrum communication method converts the several kinds of noises into very weak electric power via despreading process because the original signal is changed into a narrow bandwidth while the several kinds of noises are initially multiplied by the spread code to widen the bandwidth and remarkably reduce the interference of the noises when the receiving block multiplies the spread code for despreading.
The spread code used in such spreading and despreading processes can be used for spreading, synchronization and base station discrimination. In other words, autocorrelation and crosscorrelation processes can be executed for spreading, synchronization and base station discrimination. For detection of a desired signal, autocorrelation characteristics are required to have the maximum value when there are no time-offsets and a smaller value when time-offset values are not zero. Also, the crosscorrelation characteristics are required to have small values at all of the time-offsets for discrimination against a spread code used by a user.
In order to meet the foregoing autocorrelation and crosscorrelation characteristics, a conventional CDMA method uses a Pseudo Noise (PN) code together with a Walsh code as spread codes. The PN code satisfies required characteristics in autocorrelation, and the Walsh code satisfies required characteristics in crosscorrelation.
According to the required characteristics in the foregoing crosscorrelation, no mutual interferences exist among spread codes allocated to a number of users having one channel path but the interferences exist among the spread codes having a number of channel paths. To be more specific, the interferences are as follows:
With one channel path, the amount of mutual interference among the spread codes is determined only by the value of crosscorrelation having no time-offsets. On the contrary, with several channel paths, the amount of crosscorrelation among the spread codes is influenced not only by the crosscorrelation value having no time-offsets but also by the crosscorrelation values which have path delay times among separate channel paths as the time-offsets.
Therefore, in a multi-path channel environment having a number of channel paths which can be a real channel environment, the crosscorrelation characteristics among the spread codes have no time-offsets and the crosscorrelation values in other time-offsets become important as well.
As a result, ideally the crosscorrelation values of the spread codes are required to be 0 at all of the time-offsets. However, it is not known so far about those codes for satisfying the crosscorrelation characteristics and the autocorrelation characteristics at the same time. In other words, referring to the PN and Walsh codes in use for the conventional CDMA method, the PN codes satisfy the required characteristics of autocorrelation while failing to satisfy the required characteristics of crosscorrelation. Also, the Walsh codes fail to meet the required characteristics of autocorrelation while only partially meeting the required characteristics of crosscorrelation. So, referring to the crosscorrelation characteristics of the Walsh codes, the crosscorrelation value is 0 when the time-offsets do not exist, but is not 0 when the time-offsets are not 0.
To solve such drawbacks, one of the orthogonal codes is proposed. The code is called Large Synchronization (LS) code. The LS codes perfectly meet the autocorrelation and crosscorrelation characteristics in a certain time-offset interval. The time-offset interval for perfectly meeting the autocorrelation and crosscorrelation characteristics will be defined as an Interference Free Window (IFW).
Referring to autocorrelation characteristics in the IFW, the autocorrelation value is the maximum where no time-offsets exist, and 0 at any time-offsets in the IFW where the time-offsets are not 0. Also, according to the crosscorrelation characteristics of the LS codes, the crosscorrelation value is 0 at any time-offsets in the IFW.
As a result, in the multi-path channel environment, the interference among the spread codes allocated to users can be eliminated if the path delay time-offsets exist among the channel paths in the IFW. Therefore, the time-offset interval satisfying the foregoing autocorrelation and crosscorrelation characteristics is referred to as the Interference Free Window or IFW.
Referring the autocorrelation characteristics in the IFW, the autocorrelation value is the maximum where no time-offsets exist, and 0 at any time-offsets in the IFW where the time-offsets are not 0. In other words, when the time-offsets are restricted to the IFW interval, the autocorrelation value is the maximum when the time-offsets are 0, and 0 when the time-offsets are not 0.
However, the LS codes are known only by resultants thereof (refer to FIGS. 1A to 1F) whereas a method of generating the resultants is not known up to the present.
Meanwhile, the LS codes satisfy the autocorrelation and crosscorrelation characteristics at any time-offsets in the IFW interval.
However, the LS codes have the autocorrelation and crosscorrelation characteristics excellent in the IFW, whereas there is a disadvantage that the number of codes, which are available in use, is small thereby decreasing increase of channel capacity.
In general, referring to a set of the LS codes satisfying the autocorrelation and crosscorrelation characteristics as an orthogonal code set, a reverse proportional relation is established between the length of the IFW interval and the number of the orthogonal code set. Therefore, in the LS codes, the element number of the orthogonal code set decreases as the IFW interval increases, and on the contrary, the IFW interval decreases as the element number of the orthogonal code set increases. Therefore, a novel orthogonal spread code generating method is in request for solving the disadvantages of the LS codes thereby enabling increase of the IFW interval as well as the element number of the orthogonal code set at the same time. In convenience, a novel orthogonal spread code is called Quasi-LS (QLS) code hereafter.